Today, ABS and brake stability control systems are required for many on-road vehicles. Since the use of agricultural tractors in on-road transport operations is increasing, there is a push to implement new regulations that will require the use of ABS on some classes of agricultural tractors.
Many of the ABS systems used today for on road vehicles are based on pneumatics (trucks), or very low compliance air cooled hydraulic systems (automobiles). The brake systems used on agricultural tractors are hydraulic, but require approximately ten times more oil than an automobile. This additional oil is needed to accommodate the high running clearances needed for the oil cooled brake systems common on agricultural tractors. Others have demonstrated agricultural tractor ABS systems that utilize components from pneumatic truck brake systems. This approach is not desired as the base braking system on agricultural tractors is hydraulic. Also, the driveline of an agricultural tractor is very different than a truck or automobile. Typically, when the brakes are applied on an agricultural tractor, the mechanical front wheel drive (MFWD) clutch is closed which directly couples the front and rear axles similar to locking the transfer case on a four wheel drive truck. This configuration makes it very difficult to control ABS when braking in a condition wherein the right and left wheels are on different surfaces and have different coefficients of friction (a “split-mu” condition). An example of a split mu situation is when the vehicle right wheels are on a gravel shoulder of the road and the left wheels are on pavement. If the brakes are applied in this condition, the ABS system needs to respond differently on the right wheels from the left wheels.
It would be desirable to provide a hydraulic brake control circuit which controls the brakes on an agricultural tractor and which enables ABS and stability control functionality.